CAMBRIDGE ― By June 1 of this year, U.S. President Donald Trump had made 3,259 false or misleading claims, according to the Washington Post Fact Checker's database, which tracks and categorizes every suspect statement uttered by the president. That's an average of more than 6.5 false claims a day, up from a daily average of 4.9 untrue claims in his first 100 days, and reaching eight per day in May. Trump is clearly going for a record.Trump's supporters justify his mendacity on the grounds that "all politicians lie." Indeed they do, and a little introspection leads us to admit that all humans lie. But the amount and type of lying make a difference. Too many lies debases the currency of trust. Not all lies are born equal. Some are self-serving. A president may lie to cover his tracks, avoid embarrassment, harm a rival, or for convenience. Other presidential lies serve a loftier purpose. In some circumstances, historians even applaud the fact that a president decided to deceive the public for what he considered a larger or later good. John F. Kennedy misled the public about the role of American missiles in Turkey in the deal that ended the Cuban missile crisis in 1962; but that was certainly better for their interests than a high risk of nuclear war.A more ambiguous example occurred in 1941, before the United States entered World War II. In trying to persuade an isolationist public that Hitler's Germany was a threat, President Franklin D. Roosevelt said that a German submarine had attacked a U.S. destroyer, when in fact it was the American side that had initiated the action. In wartime, when loose lips can sink ships and secrets are crucial, Winston Churchill argued that the truth may be "so precious that she should always be attended by a bodyguard of lies."Machiavellian deception is often part of a strategy in bargaining to get a deal, and Trump claims to be a master of that art. Perhaps that explains his lies about North Korean weapons, European tariffs, and Russian President Vladimir Putin's interference in the 2016 U.S. presidential election. But his dishonesty concerning the size of the crowd at his inauguration, the payment of hush money to women, or his reasons for firing former FBI Director James Comey has nothing to do with statecraft. It is purely self-serving manipulation of others and the public.Even when a president's motives are not self-serving, he should be cautious about choosing to lie. Before he turns to lying as an instrument of statecraft, he should consider the importance of the goal, the availability of alternative means to achieve it, and whether the deception can be contained or is likely to establish a pattern.The more a leader deceives the public, the more he erodes trust, weakens institutions, and creates damaging precedents. Roosevelt's lies in 1941 were intended to awaken the American people, but he also set a precedent that Lyndon B. Johnson could use in 1964 to win congressional support for the Gulf of Tonkin Resolution, which led to a dramatic escalation of the Vietnam War. The danger is that leaders tell themselves they are lying for the public good when they are doing so for political or personal gain.Johnson did not want to seem cowardly or to be portrayed as the man who lost Vietnam. He continually lied to the American people about the progress that was being made in the war. He also wanted to keep the war limited.One of the moral benefits of a limited war is the prevention of damage through escalation. But such wars involve an element of bluffing. To maintain credibility in bargaining with the enemy, a president must maintain a relentless public optimism, which serves to misinform the public. In Johnson's case, this imperative was reinforced by his personal motives. By 1968, people were saying that the only way to tell if he was lying was to see if his lips were moving. He decided not to run again.Johnson's successor, Richard Nixon, also lied about the Vietnam War, including his expansion of it into Cambodia. This was followed by his lying about his role in the cover-up of the break-in at the Democratic Party's headquarters, which had been carried out at the behest of his administration. When this was finally revealed by the Watergate tape recordings, Nixon resigned the presidency in 1974 to avoid impeachment.The damage that Johnson and Nixon did was not only to their presidencies but also to public trust. At the beginning of the 1960s, polls showed that three-quarters of Americans had a great deal of confidence in government. By the end of the following decade, only a quarter felt that way. While the causes of the decline were complex, presidential lies played a part.Some observers, pointing to his record in the private sector, argue that Trump merely lies out of habit. Others believe that the frequency, repetition, and blatant nature of his lies reflect not habit but a deliberate political strategy to damage institutions associated with truth. Either way, Trump has eroded the credibility of institutions such as the press, the intelligence agencies, and the U.S. Department of Justice, making everything relative and playing to his extremely loyal base. Can a post-Trump America recover? It is worth remembering that Johnson and Nixon were succeeded by Gerald Ford and Jimmy Carter, who were notably more honest, and that public trust in government rose somewhat under Ronald Reagan in the 1980s. But as the sheer number of lies indicates, the U.S. has never seen a president like Donald Trump.Joseph S. Nye is a professor at Harvard and author of "Is the American Century Over?" Copyright belongs to Project Syndicate (www.project-syndicate.org).